The purpose of the pressure differential corer-carrier is to obtain long, high-quality sediment samples. To effectively design foundations for seafloor installations and anchors for massive floating structures, it is necessary to obtain certain engineering parameters of the sediments. The need for core samples at sub-bottom depths of 100 feet (30 meters) and greater has become apparent with present day ocean engineering tasks. Conventional coring techniques are not applicable to sub-bottom depths in this range. The device of this invention provides an efficient and economical means for obtaining long, high-quality cores. Novel features of this device also demonstrates principles applicable to other devices for penetrating seafloor sediment, such as piles, embeddment anchors, sediment "motors" and the like.
Various approaches have been tried to obtain long cores. These methods are either too costly or inefficient in obtaining quality cores. Drill ships which use drilling techniques to obtain quality seafloor samples from very deep sub-bottom depths are very expensive.
Another device, a seafloor, bottom-resting platform for obtaining high quality cores, samples the seafloor to a depth of 50 feet (15 meters) by using ten separate 5-foot (1.5 meter) cores taken through a cased drill hole; the hardware is complicated and expensive, and has not found application in the field.
The conventional approach of a gravity corer which uses the downward momentum of a large mass to embed the corer deep into the seafloor has been tried with only limited success for very long cores. The large mass (tens of thousands of pounds) and the corer, which is embedded in the seafloor, is difficult to retrieve without a high capacity winch on-board a surface vessel; this technique is also quite unreliable in obtaining a core of sufficient length.
The use of pressure differential techniques to drive corers themselves or to carry corers into the seafloor have been developed by others. A "bootstrap corer" was developed by Scripps Institution of Oceanography and uses a piston inside the corer to create the pressure differential. After the corer is initially set in the seafloor, the piston is pulled upward inside the corer by a cable from the surface vessel. A negative pressure differential is created on the underside of the piston and drives the corer barrel downward into the seafloor. At the same time, the sediment flows upward into the corer. However, because the sediment flows upward into the corer, it becomes considerably disturbed and is considered a poor quality sediment sample.
The "bootstrap corer" can be used as a corer-carrier where conventional corer barrels are attached to the outside of the bootstrap corer. In this manner, high quality cores can be obtained but the length of the cores are limited. Core length is dependent on the strength of the cable attaching the piston to the surface vessel and upper limit on cable strength dictates that the bootstrap corer have only a relatively small diameter compared to that attainable with the present invention. A small diameter "driver" means a relatively shallow driving depth into the seafloor.
U.S. Pat. Nos. 3,380,256 and 3,805,534 disclose methods to sink caissons and piles into the seafloor by pressure differential. These caissons and piles could function as corer-carriers. However, these prior-art patents have one major limitation. They are all essentially closed top piles. Seawater is removed from the interior to create the pressure differential. Friction forces on both the inside and outside of the pile will counter the driving force. To obtain a depth of 100 feet (30 meters) into the sediment, a large diameter pile is required (on the order of 30 feet (10 meters)) to overcome the friction forces.
This invention overcomes the above limitations.